Keywords: fossils geology paleontology petrified wood tracks trackway national park service nationalparkservice texture outdoor Fossil worm burrows in the Bright Angel Shale, in Grand Canyon National Park, Arizona. (NPS Photo by Cassi Knight, Paleontology Guest Scientist) Fossil worm burrows in the Bright Angel Shale, in Grand Canyon National Park, Arizona. (NPS Photo by Cassi Knight, Paleontology Guest Scientist) |